1. Field of the Invention
The present invention relates to a method of driving a liquid crystal display device, and more particularly to a driving method hat requires only a low voltage and consumes a reduced amount of power.
2. Description of the Related Art
A liquid crystal display (LCD) is formed by enclosing liquid crystal in the space between two transparent substrates, each having a transparent electrode formed thereon. As liquid crystal is electro-optically anisotropic, it exhibits optical properties in accordance with field strength if a desired voltage is applied across the electrodes to form an electric field in the liquid crystal layer. Utilizing such properties, a displayed image is formed as a collection of pixels, each presenting a desired brightness, by applying different voltages to the respective pixels. LCDs display an image thus formed by voltage control, providing various advantages such as reductions in size, thickness, and power consumption. As a result, LCDs are practically manufactured and widely used in office automation equipment, audio-visual devices, and the like.
FIG. 1 is an equivalent circuit diagram of such an LCD. A gate line 11 and a drain line 12 cross each other. At the intersection thereof, the LCD includes a thin film transistor (TFT) 13 serving as a switching element; a liquid crystal capacitor 14 and a storage capacitor 15, each having one electrode connected to the TFT 13; and a storage capacitor line 16 connected to the second electrode of the storage capacitor 15. The storage capacitor line 16 is shared by all the storage capacitors 15. The other electrode of the liquid crystal capacitor 14 is provided as a common electrode formed on a substrate opposite to the substrate on which the TFT 13 is disposed with liquid crystal interposed therebetween, and is connected to a common line 17.
FIG. 2 illustrates waveforms of signal voltages driving the LCD shown in FIG. 1. During an ON period, a gate voltage VG applied to the gate line 11 attains a high level. During this period, the TFT 13 is turned on, resulting in drain-source conduction. As a result, a source voltage VS becomes equal to a drain voltage VD applied to the drain line 12, and is applied to said one electrode of each of the liquid crystal capacitor 14 and the storage capacitor 15. At the beginning of an OFF period, the gate voltage VG falls to a low level, turning of f the TFT 13, to thereby determine the source voltage VS.
At the instant the gate voltage VG falls from the high level to the low level, the source voltage VS falls by an amount xcex94VS due to capacitance coupling, and is retained as a pixel voltage VP. Meanwhile, the respective other electrodes of the liquid crystal and storage capacitors 14 and 15 receive the same common voltage Vcom from the storage capacitor line 16 and the common line 17. The resulting difference in voltage between the common voltage Vcom and the pixel voltage VP serves as a voltage VLC for driving liquid crystal that is applied to the liquid crystal and storage capacitors 14 and 15. The pixel voltage VP is maintained by off-resistance of the TFT 13 until the TFT 13 is turned on again to charge the capacitor to a different voltage in the next field, though it is decreased by an amount xcex94VK due to leakage current. As the storage capacitor 15 is connected in parallel to the liquid crystal capacitor 14, exactly the same voltage is applied thereto, so that these capacitors 14 and 15 contribute to a reduction in the amounts xcex94VS and xcex94VK by increasing their combined capacitance.
Usually, the polarity of the voltage applied to the liquid crystal capacitor 14 is inverted every frame period, field period, line period, or the like, in order to prevent deterioration of the liquid crystal. This method is referred to as a common inversion driving method, in which the polarity of the common voltage Vcom is inverted at the opposite timing to the drain voltage VD. Consequently this method achieves a decrease in the amplitude of the drain voltage VD and a reduction in a power supply voltage for a drain driving circuit, contributing to a decrease in power consumption.
However, according to such a common inversion driving method, the common voltage Vcom is an alternating-current voltage signal, and is applied in common to all the liquid crystal and storage capacitors 14 and 15. As a result, considerable wiring capacitance of the storage capacitor line 16 and of the common line 17 is necessary and a large amount of current flows during the change in voltage. This increases overall power consumption of the device, including the power consumed by the common electrode and the storage capacitor electrode.
The present invention was conceived to solve the above-described problems, and aims to provide a display device that consumes less electric power than current devices.
In order to achieve the above object, the present invention is directed to a method of driving a liquid crystal display device comprising a pixel electrode formed on a first substrate, a switching element connected to said pixel electrode, a common electrode formed on a second substrate, liquid crystal provided between said pixel electrode and said common electrode, and a storage capacitor utilizing said pixel electrode as one electrode.
This method comprises the step of applying, to the other electrode of said storage capacitor, a storage capacitor voltage that is changed from a low level to a high level immediately after said switch element is turned off during a period in which a voltage of said pixel electrode is higher than that of said common electrode, and that is changed from the high level to the low level after said switching element is turned of f during a period in which the voltage of said pixel electrode is lower than that of said common electrode.
According to one aspect of the present invention, the voltage of said common electrode is a direct-current voltage.
By thus changing the level of the storage capacitor voltage in accordance with a relationship between the voltages of the pixel electrode and the common electrode, the voltage of the pixel electrode can be shifted. As described above, the storage capacitor voltage attains a high level during a period in which the pixel electrode voltage is higher than the common electrode voltage and said switching element is off, and attains a low level during a period in which the pixel electrode voltage is lower than the common electrode voltage and said switching element is off. Consequently, a sufficiently large voltage can be applied to the liquid crystal capacitor even if an amplitude of the voltage of a display signal supplied to each switching element is decreased.
Another aspect of the present invention is directed to a method of driving a liquid crystal display device comprising a pixel electrode formed on a first substrate, a switching element connected to said pixel electrode, a common electrode formed on a second substrate, liquid crystal provided between said pixel electrode and said common electrode, and a storage capacitor utilizing said pixel electrode as its one electrode. The method according to this aspect then comprises the steps of applying a direct-current voltage to said common electrode and applying, to the other electrode of said storage capacitor, a storage capacitor voltage the level of which changes during a period in which said switching element is off.
According to a still further aspect of the present invention, said storage capacitor voltage changes from a low level to a high level during a period in which said switching element is off and the voltage of said pixel electrode is higher than that of said common electrode, and changes from the high level to the low level during a period in which the voltage of said pixel electrode is lower than that of said common electrode.
According to a yet further aspect of the present invention, the storage capacitor voltage changes from the low level to the high level, or from the high level to the low level, immediately after said switching element is turned off.
By thus changing the level of the storage capacitor voltage and using a direct-current voltage as the voltage of the common electrode, no current flows through said common electrode having a large wiring capacitance. Therefore, the above-described driving method allows reduction in power consumed by the liquid crystal display device.
According to a further aspect of the invention, the present invention is directed to a method of driving a liquid crystal display device comprising a pixel electrode formed on a first substrate, a switching element connected to said pixel electrode, a common electrode formed on a second substrate, liquid crystal provided between said pixel electrode and said common electrode, and a storage capacitor utilizing said pixel electrode as its one electrode, said method comprising the step of applying, to the other electrode of said storage capacitor, a storage capacitor voltage the level of which changes during a period in which said switching element is off.
By thus employing a method in which the level of said storage capacitor voltage is changed, a liquid crystal driving voltage applied across said pixel electrode and said common electrode can be increased without changing the voltage of said common electrode with a large wiring capacitance, and without increasing the voltage of a display signal supplied to said pixel electrode via said switching element. In addition, because the level of the storage capacitor voltage is changed during a period in which said switching element is off, it is possible to ensure complete application of the display signal applied to said pixel electrode via said switching element during an ON period.
According to a further aspect of the present invention, in any of the above described methods of driving a liquid crystal display device, the voltage of said common electrode is set lower than a central voltage of the display signal applied to said switching element by a prescribed voltage.
Further, in any of the above methods of driving a liquid crystal display device, a potential difference between said central voltage and said common electrode voltage is a potential difference in accordance with an amount of change in the voltage of said pixel electrode connected to said switching element exhibited when said switching element is turned off.
By thus establishing said common electrode voltage to satisfy the above-described conditions, the amplitude of the voltage applied to the liquid crystal capacitor is more efficiently increased, to thereby further reduce the amount of power consumed by the device.